1. Field of the Invention
The present invention relates to the measurement of the progress of a chemical reaction that generates an electroactive reaction product that is subsequently detected at an electrode amperometrically or coulometrically. The method is useful in applications where it is desirable to follow the progress of a chemical reaction, particularly in sensor applications where the progress of the reaction of an analyte can be useful in determining the analyte concentration.
2. Description of the Related Art
In amperometric electrochemistry the current flowing at the electrode can be used as a measure of the concentration of electroactive species being reacted electrochemically at the working electrode. In coulometry the current flowing at the electrode is integrated over time to give a total amount of charge passed which yields a measure of the amount of electroactive material reacted at the working electrode. The current flowing (or charge passed at any time) at the electrode is dependent upon the rate of transfer of the electroactive species to the working electrode. When a significant concentration of electroactive species is situated close to the electrode and an electrical potential is applied to the electrode sufficient to electrochemically react the electroactive species at the electrode/solution interface, initially a higher current will flow which will diminish with time. For an isolated electrode, where the potential applied to the electrode is sufficient to react the electroactive species effectively instantaneously upon arriving at the electrode and the transfer of electroactive species to the electrode is controlled by diffusion, the current will follow a curve known in the art as the Cottrell Equation. According to this equation the current varies inversely with the square root of time. This yields a current which decays with time as the electroactive species that reacts at the electrode becomes depleted close to the electrode and so electroactive species has to travel from further and further away to reach the electrode as time progresses.
If in addition to the electrochemical reaction of the electroactive species at the electrode the electroactive species is being generated close to the working electrode by a chemical reaction, the form of the current flowing at the electrode becomes complex. The electrode reaction tends to decrease the concentration of electroactive species close to the working electrode whereas the chemical reaction tends to increase the concentration of the electroactive species in this region. The time dependent behavior of these two processes therefore mix and it is difficult to measure the chemical reaction kinetics from the current flowing (or charge passed) at the electrode.
For this reason, in the published literature, the rates of chemical reactions are not generally measured electrochemically except in specialized applications using specialized equipment. An example of such equipment is known in the art as a rotating ring/disc electrode. This apparatus is only applicable to relatively fast reaction kinetics and requires that the electrode be rotated at a known controlled rate with well characterized liquid hydrodynamics.